allow "cover" in rule tpb13 for single-doc HTML validation
allow "cover" in rule tpb13 for single-doc HTML validation

nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator